We will identify which protein in our purified complex is the poly C polymerase by using agents which can be cross-linked to the enzyme. We will use the photoaffinity analogue of GTP, 8-azidoquanosine triphosphate labeled with 32P to label the substrate-binding site, and in separate experiments, use 14C-phenylglyoxal to label the template-binding site. The protein to which these agents are covalently linked will be identified during polyacrylamide gel electrophoresis. We will determine the ability of our purified complex to bind and copy in vitro-synthesized reovirus mRNA's. We will determine the capacity to assemble our purified complex after shift-up to nonpermissive temperature of cells infected with reovirus mutants known to be defective in the synthesis of double-stranded RNA at the nonpermissive temperature.